1. Field of the Invention
The present invention relates to a semiconductor light emitting element. More particularly, the present invention relates to a structure of, and a method for fabricating, a quadruple alloy light emitting diode (LED) made of a quadruple alloy material of AlGaInP for constituting a high-luminescence LED which emits light in a red to green band.
2. Description of the Related Art
In recent years, a high-luminance quadruple alloy LED made of AlGaInP has become the object of particular attention as a light emitting element for various types of display devices for indoor use and outdoor use. A quadruple alloy material allows for the fabrication of an LED which emits light in a wide visible wavelength region ranging from a red to green band.
A typical structure of a conventional quadruple alloy LED 1100 for a yellow band is shown in FIGS. 7A and 7B: FIG. 7A is a perspective view thereof; and FIG. 7B is a schematic cross-sectional view thereof.
In this structure, an n-(Al0.7Ga0.3)0.5In0.5P cladding layer 51 (doped with Si, carrier concentration: about 5xc3x971017 cmxe2x88x923, thickness: about 1.5 xcexcm), a non-doped (Al0.3Ga0.7)0.5In0.5P active layer 52 (thickness: about 0.7 xcexcm), a p-(Al0.7Ga0.3)0.5In0.5P cladding layer 53 (doped with Zn, carrier concentration: about 5xc3x971017 cmxe2x88x923, thickness: about 1.5 xcexcm), a p-Al0.7Ga0.3As current diffusion layer 54 (doped with Zn, carrier concentration: about 3xc3x971018 cm3, thickness: about 5 xcexcm), and a p-GaAs ohmic contact layer 55 (doped with Zn, carrier concentration: about 3xc3x971018 cmxe2x88x923, thickness: about 0.5 xcexcm) are sequentially formed in this order on an n-GaAs substrate 50 by metal organic chemical vapor deposition (MOCVD). In addition, lower and upper electrodes 56 and 57 are formed on the reverse surface of the substrate 50 and on the top surface of the grown layered structure, respectively. The upper electrode 57 on the top surface of the grown layered structure, as well as the p-GaAs ohmic-contact layer 55, have been patterned so as to be in a circular shape in the center region of the top surface of the structure. Portions of the upper electrode 57 and the pGaAs ohmic contact layer 55 have been removed by performing an etching, leaving the circular-shaped portions remaining in the center region.
An axial luminous intensity (unit: candela (cd)) of a molded LED element is one of the indices representing the luminescence of the LED. In the conventional LED 1100 shown in FIGS. 7A and 7B, when the axial spread angle of the emitted light is about xc2x14 degrees with an operating voltage of about 2.0 V and a drive current of about 20 mA, the axial luminous intensity is about 8 cd.
The apparent axial luminous intensity is increased as the light concentration characteristics of an LED are improved (i.e., as the axial spread range of the emitted light is smaller). Moreover, an LED having improved light concentration characteristics can be advantageously used for communication applications.
Another conventional LED 1200, for communication, is shown in FIGS. 8A and 8B: FIG. 8A is a perspective view thereof; and FIG. 8B is a schematic cross-sectional view taken along the line 8B-8Bxe2x80x2 of the LED 1200 shown in FIG. 8A. The conventional LED 1200 shown in FIGS. 8A and 8B is an AlGaInP alloy system LED for a yellow band and has the following structure.
As shown in the schematic cross-sectional view in FIG. 8B, an n-(Al0.7Ga0.3)0.5In0.5P cladding layer 51 (doped with Si, carrier concentration: about 1xc3x971018 cmxe2x88x923, thickness: about 1.0 xcexcm), a non-doped (Al0.3Ga0.7)0.5In0.5P active layer 52 (thickness: about 0.6 xcexcm), a p-(Al0.7Ga0.3)0.5In0.5P cladding layer 53 (doped with Zn, carrier concentration: about 1xc3x971018 cmxe2x88x923, thickness: about 1.0 xcexcm), an n-(Al0.7Ga0.3)0.5In0.5P current constriction layer 58 (doped with Si, carrier concentration: about 2xc3x971018 cmxe2x88x923, thickness: about 0.4 xcexcm), a p-Al0.7Ga0.3As current diffusion layer 54 (doped with Zn, carrier concentration: about 3xc3x971018 cmxe2x88x923, thickness: about 6 xcexcm), and a p-GaAs ohmic contact layer 55 (doped with Zn, carrier concentration: about 3xc3x971018 cm3, thickness: about 0.5 xcexcm) are sequentially formed in this order on an n-GaAs substrate 50 by MOCVD.
The center region of the n-(Al0.7Ga0.3)0.5In0.5P current constriction layer 58 has been etched away in a circular shape to form a light emitting region, and the p-Al0.7Ga0.3As current diffusion layer 54 is re-grown over the current constriction layer 58 including the etched and removed center region thereof. The reference numeral 59 denotes the re-growth interface.
In addition, lower and upper electrodes 56 and 57 are formed on the reverse surface of the substrate 50 and on the top surface of the grown layered structure, respectively. The upper electrode 57 and the p-GaAs ohmic contact layer 55 are formed in a doughnut shape in which the center regions thereof are etched away so as to have openings of the same size and shape as those of the etched and removed region of the current constriction layer 58.
In this conventional LED element 1200, an injected current flows in a concentrated manner into the center region, so that the reduced spot size of emitted light can be realized. As a result, the light concentration characteristics of the resulting element, which has been molded with a resin, can be improved and the axial luminous intensity thereof can be increased.
However, in the conventional LED 1200 shown in FIGS. 8A and 8B, the p-Al0.7Ga0.3As current diffusion layer 54 is re-grown on the underlying p-(Al0.7Ga0.3)0.5In0.5P cladding layer 53 containing Al. Thus, oxygen is likely to be absorbed into the re-growth interface 59 (see FIG. 8B), resulting in various losses such as adversely increased resistance and non-radiative recombination of injected carriers.
The typical operational characteristics of such a conventional LED 1200 are as follows: the axial spread angle is about xc2x12 degrees and the luminescence is about 16 cd with the operating voltage of about 3.0 V when a power of about 20 mA is supplied thereto. As compared with the conventional LED 1100 shown in FIGS. 7A and 7B (which is made of the same quadruple alloy material and has the axial spread angle of about xc2x14 degrees and the luminescence of about 8 cd with the operating voltage of about 2.0 V when a power of about 20 mA is supplied thereto), the axial luminous intensity of the LED 1200 shown in FIGS. 8A and 8B is increased only by as little as twofold while the operating voltage is considerably increased. In the element 1200 shown in FIGS. 8A and 8B, the luminescence has been expected to be increased fourfold (i.e., about 32 cd) since the axial spread angle thereof is decreased to about xc2xd of that of the element 1100 shown in FIGS. 7A and 7B.
In order to solve such problems as set forth above, another conventional semiconductor light emitting element 1300 having such a structure as that shown in FIG. 9 has been suggested. The shape of the current constriction layer and the electrode on the top surface of the grown layered structure of the semiconductor light emitting element 1300 shown in FIG. 9 is the same as that of the element 1200 shown in FIGS. 8A and 8B.
As shown in the schematic cross-sectional view in FIG. 9, an n-(Al0.7Ga0.3)0.5In0.5P cladding layer 51 (doped with Si, carrier concentration: about 1xc3x971018 cmxe2x88x923, thickness: about 1.0 xcexcm), a non-doped (Al0.3Ga0.7)0.5In0.5P active layer 52 (thickness: about 0.6 xcexcm), and a p(Al0.7Ga0.3)0.5In0.5P cladding layer 53 (doped with Zn, carrier concentration: about 1xc3x971018 cmxe2x88x923 thickness: about 1.0 xcexcm) are sequentially formed in this order on an n-GaAs substrate 50 by MOCVD. Next, unlike the conventional element 1200 shown in FIGS. 8A and 8B, a p-GaInP layer 60 (doped with Zn, carrier concentration: about 1xc3x971018 cmxe2x88x923, thickness: about 100 xc3x85) not containing Al is formed on the p-(Al0.7Ga0.3)0.5In0.5P cladding layer 53 in the element 1300 shown in FIG. 9. Since the layer 60 functions as the underlying layer during the re-growth process, oxygen is less likely to be absorbed into the re-growth interface 59, and conditions of the re-growth interface 59 can be improved as compared with the conventional example 1200 shown in FIGS. 8A and 8B.
The remaining part of the element 1300 shown in FIG. 9 is the same as that of the element 1200 shown in FIG. 8B. Specifically, an n-(Al0.7Ga0.3)0.5In0.5P current constriction layer 58 (doped with Si, carrier concentration: about 2xc3x971018 cm3, thickness: about 0.4 xcexcm), a p-Al0.7Ga0.3As current diffusion layer 54 (doped with Zn, carrier concentration: about 3xc3x971018 cmxe2x88x923 thickness: about 6 xcexcm), and a p-GaAs ohmic contact layer 55 (doped with Zn, carrier concentration: about 3xc3x971018 cmxe2x88x923, thickness: about 0.5 xcexcm) are sequentially formed in this order on the p-GaInP layer 60.
The semiconductor light emitting element of the present invention includes: a compound semiconductor substrate having a first conductivity type; a light emitting layer; a compound semiconductor interface layer having a second conductivity type and not containing Al; and a current diffusion layer having the second conductivity type and being made of a compound semiconductor not containing Al.
A current constriction layer having the first conductivity type and being made of a compound semiconductor not containing Al may be further provided between the compound semiconductor interface layer and the current diffusion layer.
A carrier concentration of the current diffusion layer may increase from a region thereof over the compound semiconductor interface layer toward a region thereof under an upper electrode.
The light emitting layer may have a double heterostructure in which an AlGaInP or AlInP cladding layer having the first conductivity type, an AlGaInP or GaInP active layer, and an AlGaInP or AlInP cladding layer having the second conductivity type are sequentially formed in this order.
A semiconductor layer providing a light reflection function may be further provided between the compound semiconductor substrate and the light emitting layer.
A band gap adjustment layer having an intermediate band gap may be further provided between the light emitting layer and the compound semiconductor interface layer.
A buffer layer may be further provided between the compound semiconductor substrate and the light emitting layer.
The compound semiconductor interface layer, the current constriction layer, and the current diffusion layer may be made of a GaP compound material. Preferable, the compound semiconductor interface layer, the current constriction layer, and the current diffusion layer are made of a GaP compound material of the same composition.
The current constriction layer may have an opening portion in a center portion of the semiconductor light emitting element.
Preferably, a thickness of the compound semiconductor interface layer is equal to or smaller than about 3.0 xcexcm.
Preferably, the compound semiconductor interface layer has a carrier concentration in a range from about 2xc3x971016 cmxe2x88x923 to about 2xc3x971018 cmxe2x88x923, and the current diffusion layer has a carrier concentration of about 2xc3x971018 cmxe2x88x923 or more.
According to another aspect of the present invention, the method for fabricating such a semiconductor light emitting element having the above-mentioned features is provided. The method includes the steps of: forming the light emitting layer and the compound semiconductor interface layer containing no Al on the compound semiconductor substrate; and forming the current diffusion layer over the compound semiconductor interface layer. A growth process is suspended at a predetermined time so that a re-growth interface is located on a surface of the compound semiconductor interface layer.
Hereinafter, the functions and/or the effects to be attained by the present invention will be briefly described.
The present inventors found that the operating characteristics of the LED 1300 shown in FIG. 9 are still unsatisfactory. Specifically, the LED 1300 has the axial spread angle of about +2 degrees and a luminescence of about 24 cd with the operating voltage of about 2.4 V. The reasons for the above are presumably as follows.
In the LED 1300, the underlying layer for the re-growth process is the p-GaInP layer 60, while the re-grown layer 54 is the p-Al0.7Ga0.3As current diffusion layer 54. These layers 60 and 54 have different Group V elements, i.e., arsenic (As) in the layer 54 and phosphorus (P) in the layer 60. As a result, it is difficult to stoichiometrically match the layers 54 and 60. Furthermore, the conditions of the re-growth interface 59 are still satisfactory, resulting in a high-resistance layer. Consequently, the injected carriers are also lost to a large degree.
The present invention has been made in view of the above-mentioned findings by the present inventors.
When the present invention is applied to the semiconductor light emitting element of an AlGaInP alloy system, the semiconductor light emitting element of the present invention may include: a compound semiconductor substrate having the first conductivity type (n-GaAs); a buffer layer (n-GaAs); a light emitting layer (cladding layer/active layer/cladding layer); a compound semiconductor interface layer having the second conductivity type and not containing Al (p-GaP); a current constriction layer having the first conductivity type and being made of a compound semiconductor not containing Al (nGaP); and a current diffusion layer having the second conductivity type and being made of a compound semiconductor not containing Al (p-GaP).
The growth process is suspended so that the re-growth interface is located on the surface of the compound semiconductor interface layer which contains no Al. Thus, oxygen is not absorbed into the re-growth interface.
In addition, since the layers made of the same GaP material are formed with the re-growth interface interposed therebetween, no interface level resulting from a stoichiometric difference is generated.
Consequently, the present invention can provide a semiconductor light emitting element realizing a low resistance and a high luminescence.
Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor light emitting element realizing a low resistance and a high luminescence in which no oxygen is absorbed into the re-growth interface between an underlying layer and a re-growth layer, preventing interface levels which result from the stoichiometric difference from being generated, and (2) providing a method for fabricating such a semiconductor light emitting element.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.